Means for maintaining the substitutability of lng



ch. 7,, 196? Ramos-r012 ETAL 3,302,413

MEANS FUR MAINTAINING THE SUBSTITUTABILITY OF LNG 2 Sheets-Sheet 1 Filed April 16, 1965 High Pressure l2 l w Low Pressure Disfribufion Line 9 Ill 1 Q "2 F A I m w e v w 7 4 W 6 5 6 s Ar A% D.. 4 er! 3 9p nr mm M b MD 3 L X% 3 EN m P IT 0 9 CA N N Cylinder For Make-Up Alrerncng Make-Up m m m m Russell C.Procfor Roger W. Parrish ATTORNEY Feb. 7, 1967 R. c. PROCTOR ETAL 3,302,416

MEANS FOR MAINTAINING THE SUBSTITUTABILITY 0F LNG Filed April 16, 1965 2 Sheets-Sheet 2 Distribution me LNG" Feed Gas 1 '1 g l p u J I l N compressor g l 0 Exchange w.

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WILCOHTYOI Line 59 N -LNG k Exchanger 5i LIV -LNG Subcooler INVENTORS Russel! C Proctor Roger W. Parrish BY j hg ATTORNEY United States Patent 3,302,416 MEANS FOR MAINTAINING THE SUBSTITUT- ABILITY OF LNG Russell C. Proctor, Leawood, Kans., and Roger W. Parrish, Independence, Mo., assignors to Conch International Methane Limited, Nassau, Bahamas, a Bahamian company Filed Apr. 16, 1965, Ser. No. 448,639 8 Claims. (CI. 62-45) This invention relates to a system for storing LNG (liquid natural gas) in such a manner as to maintain its substitutability. LNG is a mixture of hydrocarbon gases, all of which have varied boiling points. Under conditions of ordinary storage in a tank, from which some of the product is allowed to boil off, fractionation of the mixture of the gases takes place in the storage tank, clue to these difierent boiling points. The higher boiling point constit=uents vaporize less readily than those of lower boiling point, which produces a boil-off product having a lower B.t.u. content and a diiferent composition from the liquid inventory remaining in the tank.

It is possible, by means of some schemes which have been proposed, to make up for the above effect by manipulating the gas fractions so that the product has a constant B.t.u.- However, this is not suflieient for many customers who .require that the gas be substitutable. The substitutability of the gas is measured by the lifting limit, flash-back limit and yellow tip limit, which factors are defined by the A.G.A. as the principal factors in determining the substitutability of a gas. The present invention has for its major object to maintain the substitutability of a product which is stored prior to use in liquid form at substantially atmospheric pressure, as a mixture of liquid gases of which the constituents have different boiling points. This is accomplished, according to the invention, primarily by adding sufiicient refrigeration to the stored liquid gases to equal the heat leak into storage and thereby prohibit any boiloif at all from occurring. Without boiloff, there can be no composition change, therefore the liquefied natural gases remain completely substitutable with pipeline gas, since they have the same composition.

A major object of the invention is to add refrigeration (i.e., remove heat) from the stored liquid gases in a highly efficient manner and at a relatively low cost both in energy and in equipment.

According to the present invention, the above and other objects are accomplished by providing a refrigeration source, external to the stored gases, subcooling limited amounts of the stored liquid gas from selected locations of a storage volume sufiicient to make up the heat leakage into the system, and returning said subcooled liquid to preferred locations of the storage space in such manner as to provide relatively uniform temperature conditions throughout the stored liquid with minimum disturbance of the stratified vapors above the liquid surface. More specifically, in a preferred form of the invention, liquid is withdrawn from the storage reservoir from a region near the bottom of the reservoir, the withdrawn liquid is subcooled, and the liquid is returned to the main liquid body in such fashion as to maintain the temperature of the liquid substantially uniform throughout its volume and at a level such that no boiloff can occur. Another feature of the invention resides in effecting the makeup refrigeration in the vapor space of the tank for maximum efliciency. According to the invention, the total volume of liquid is not subcooled, but is maintained at equilibrium temperature at substantially ambient atmospheric pressure, so that the difference between the external and internal pressures on the storage reservoir are reduced to a minimum.

Another advantage of the invention is that the necessary 3,302,416 Patented Feb. 7, 1967 heat exchange is accomplished between liquids rather than gases, which reduces the time lag and permits closer temperature control.

Still another advantage is the elimination of the vapor return line from the tank, which is an expensive line, and must be very long, usually in the order of 250-300 feet, for reasons of established code clearances.

The major objects of the invention are accomplished by subcooling selected portions of the LNG by heat exchange against nitrogen. In one form, an N -LNG subcooler is employed to subcool the LNG in an amount normally sufiicient to make up for the heat leak into the tank while inventory is being added to the tank. However, during periods of holding, when the tank is full and no inventory is being added, an amount of subcooled make (input liquid) is diverted through a subcooler inside the storage tank, where it is heat exchanged with an amount of inventory piped through the subcooler. In accordance with the invention, the sub-cooled inventory is preferably returned below the liquid level under normal operation, in order to maintain nearly uniform temperature conditions throughout the stored liquid; however, if more immediate response is needed to a rising pressure in the tank, means are provided for diverting the sub-cooled liquid through a valve and spraying it into the vapor space to more abruptly reduce the pressure within the tank.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

FIG. 1 is a schematic flow chart showing the principle of the invention;

FIG. 2 is a schematic sectional view of a novel subcooler intended for use Within the LNG reservoir; and

FIG. 3 is a flow chart of a modified system according to the invention.

Referring to FIG. 1, LNG is supplied as high-pressure feed gas on line 2, and is liquefied prior to storage by means of several heat exchange steps, which may be conventional, and which are shown in general fashion as a C (propane) loop 3, a C (ethane, ethylene) loop 4, a C (methane) heat-exchange step 6 at distribution pressure, and a C (methane) heat-exchange step 7 at atmospheric pressure, since the fluid is stored in tank 8 at atmospheric pressure. A portion of the input is diverted on line 9, through valve 11, and supplied through the preceding heat-exchange steps in reverse, directly to the low pressure distribution line 12. This is done in order to take advantage of the reduction of pressure in valve 11, with its resultant refrigeration effect. An additional portion Olf the input is diverted on line 10 through valve '15 and supplied through the heat-exchange steps in reverse to a compressor 20 which compresses this gas to the pressure in the low pressure distribution line 12. The product in line 2 is then passed through line 13 and a further pressure reduction valve 14 into storage with a small flash to atmospheric pressure. A major advantage of storing the fluid at atmospheric pressure, as is well known, is that the storage reservoir 8 need not be built to withstand any gas pressure either internal or external, which would be very difficult to do with the large tanks used for this purpose, often over feet in diameter. In usual practice, the vapor pressure in the tank is maintained very slightly above atmospheric, to avoid any danger of air entering the vapor space and producing a dangerous mixture. For this purpose, a vent line is usually employed, and upon any rise in pressure due to heat leakage into the tank, the excess vapor is simply vented off to be reliquefied or used as fuel after compression.

In order to provide the necessary refrigeration to offset heat loss in the tank, and eliminate handling these low pressure vapors, a nitrogen refrigeration circuit is employed, the nitrogen in line 16 being compressed to a suitable pressure by means of compressor 17, cooled in intercooler 18, and passed in line 19 through the heat exchangers 3, 4, 6 and 7 as previously shown. The nitrogen emerges on line 21 at 248 F. and 300 p.s.i.a., is passed through a pressure reduction valve 22 to line 23, and thence to coil 24 of a special sub-cooler unit, and thence back to line 26 and the respective heat exchangers back to line 16, where it is recycled through compressor 17.

Subcooler 25 is supplied with LNG from near the bottom of the reservoir through pump 27 and line 28. The sub-cooled inventory emerges on line 29 and is delivered, preferably near the central portion of the fluid in the tank on line 35, which may be a tangential nozzle, or a series of nozzles, in order to ensure fairly thorough mixing of the sub-cooled liquid with the liquid in the tank, the amount of sub-cooled liquid thus supplied being regulated so as to maintain the desired equilibrium conditions in the tank. This can be done in any suitable or known manner, and is indicated generally by the provision of a pressure control device 33, responsive to the pressure in the tank to control the valve 22 through control line 34.

A cylinder of nitrogen under pressure, 41, is used to supply makeup nitrogen through valve 42 to the nitrogen refrigeration cycle as needed. However, when the stored gas contains an appreciable amount of nitrogen, since this has a lower boiling point than the other constituents of the LNG, the vapor in the tank will be very rich in nitrogen, and advantage can be taken of this by using this vapor on line 43, controlled by valve 44, as an alternative source of makeup gas.

Referring to FIG. 3, the system is the same as shown in FIG. 1 down to exchanger 7, but at this point, the nitrogen line 16 is passed through the N -LNG heat exchanger 57, then through another heat exchanger 51, which serves as an N LNG subcooler for supplying subcooled LNG on line 52 to the tank through valve 53 and through line 61 to subcooler unit 25', which corresponds in function to subcooler unit 25 of FIG. 1, except that its cooling coil 24' contains subcooled LNG instead of N Subcooler 25' is supplied with inventory (LNG) from a point near the bottom of the tank, through pump 27 and line 28, and the LNG, after being suitably subcooled, emerges on line 29 and is normally passed by valve 31 to line 32, which terminates preferably near the midpoint of the reservoir, usually in a tangential nozzle so that the subcooled LNG may be very thoroughly mixed with the main body of the liquid, thereby providing sufficient rerigeration to exactly offset normal heat leakage into the tank, and to maintain the mass of LNG in the reservoir at equilibrium temperature corresponding to atmospheric pressure. This condition is automatically maintained by means of a pressure control unit 33, which through control line 34 controls valve 36 to provide additional refrigeration if the pressure rises, and vice versa. If for any reason the pressure rise should be too rapid and the pressure tend to rise to an undesired point, an additional control line 37 is provided to operate on valve 31 to divert the subcooled inventory from line 29 through line 38 to a number of spray nozzles 39 to quickly reduce the temperature of the vapor in the reservoir and thus maintain its pressure at the safe level. Since this last operation is less desirable than providing the fluid directly to the main body of the reservoir via line 32, it is not normally employed. It is particularly important to note that the above system enables the entire body of liquid to be maintained at substantially equilibrium temperature at atmospheric pressure. If the liquid were to be sub-cooled, then the vapor pressure within the reservoir would be less than atmospheric, and the reservoir, and particularly its roof, would have to be built sufficiently strong to withstand this inward pressure, which would require a much more expensive structure.

It will be noted that the system of FIG. 3 does not employ the nitrogen directly in the subcooler 25', but that the coil 24 uses instead subcooled LNG which is brought to the desired low temperature by heat exchange with the nitrogen refrigeration system. The subcooler 25 is not normally employed during periods when the tank is in use, that is, when inventory is being constantly supplied; during such periods, the desired equilibrium conditions can be maintained by subcooling the make in subcooler 51, from which it emerges on line 52 at the desired low temperature, and is passed through valve 53 into the storage reservoir tank 8 at a temperature sufficiently low to make up for a normal heat leakage into the tank, and to maintain the desired equilibrium conditions. However, when the tank is used to store LNG for extended periods, the equilibrium conditions cannot be maintained by this means, since at such times there can be no input into the tank if the tank is full. When this condition occurs, valve 54, which is normally closed, is opened, and normally open valve 53 is closed, thus establishing a LNG circuit through subcooler 25', line 57, and N -LNG exchanger 57, line 58, through exchangers 7, 6, 4 and 3 to the distribution line. Subcooler 25' operates similarly to subcooler 25 of FIG. 1, except that its coil uses LNG instead of nitrogen gas directly. The stages preceding exchanger 57 can be exactly the same as those shown in FIG. 1.

The primary advantage to this system is in eliminating the large vapor line between the storage and the liquefaction plant and substituting a much smaller liquid line 57.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement Within the scope of our invention as defined in the appended claims.

We claim:

1. System for maintaining the substitutability of LNG comprising (a) a large-scale storage reservoir for LNG having a vapor space near the top thereof,

(b) means for supplying LNG to said reservoir for storage at substantially atmospheric pressure,

(c) a heat-exchange unit in said vapor space, said unit having a supply of refrigerant external to the reservoir at a lower temperature than the stored LNG,

(d) means for withdrawing LNG from a point near the bottom of the reservoir and passing it through said heat-exchange unit to subcool the withdrawn LNG,

(e) means for returning the subcooled Withdrawn LNG to the body of LNG in the reservoir at a rate sufficient to offset heat leakage into the reservoir and to maintain equilibrium conditions in the res ervoir so as to prevent boil-off.

2. The invention according to claim 1, wherein said external refrigerant is N 3. System for maintaining the substitutabilty of LNG comprising (a) a large-scale storage reservoir for LNG having a vapor space near the top thereof,

(-b) a high-pressure line for supplying natural gas,

(c) multi-refrigerant means for reducing the temperature of said gas,

(d) means for reducing the pressure of said natural gas and supplying it to said reservoir in liquefied form as LNG at substantially atmospheric pressure,

(e) a nitrogen refrigeration circuit for subcooling the supplied LNG to a lower temperature than the LNG in said reservoir,

(f) heat-exchange means for subcooling the natural gas with N such that the input to said reservoir is at a temperature sufficient to olfset heat leakage into the reservoir and prevent boil-off or increase in pressure in the vapor space above the liquid during periods when the reservoir is being supplied with input,

(g) a heat-exchange unit in said vapor space, and

means for supplying said unit with subcooled refrigerant from said refrigeration circuit,

(h) means for withdrawing LNG from a point near the lower portion of the reservoir and passing it through said heat-exchange unit to cool the withdrawn LNG,

(i) means for returning the cooled withdrawn LNG to the body of LNG in the reservoir at a rate sufficient to offset heat leakage into the reservoir and maintain equilibrium conditions in the reservoir so as to prevent boil-oft or increase in tank pressure during periods when no input is supplied to the reservoir.

4. The invention according to claim 3, and pressurecontrol means for controlling the operation of said heatexchange unit so as to maintain the pressure in the tank at-a point above atmospheric pressure but within the design pressure of the tank.

5. The invention according to claim 3, and pressurecontrol means for controlling the operation of the LNG subcooler and the temperature of the input to the reservoir so as to maintain the pressure in the tank above atmospheric pressure but within the design pressure of the tank.

6. The invention according to claim 3, including additional means for spraying subcooled LNG into the vapor space of the reservoir, and pressure-controlled means for activating said last additional means only upon an excessive rise in pressure in the vapor space so as to rapidly reduce and control the pressure in the vapor space.

7. The invention according to claim 1, said means for returning the subcooled withdrawn LNG to the body of LNG including, means for delivering the subcooled LNG to a region of the reservoir approximately midway between the top and bottom of the reservoir.

8. System for maintaining the substitutability of LNG comprising (a) a large-scale storage reservoir for LNG having a vapor space near the top thereof,

(b) means for supplying LNG to said reservoir for storage at substantially atmospheric pressure,

(c) a heat-exchange unit in said vapor space, said unit having a supply of external N refrigerant at a lower temperature than the stored LNG,

(d) means for withdrawing LNG from a point near the bottom of the reservoir and passing it through said heat-exchange unit to subcool the withdrawn LNG,

(e) means for returning the subcooled withdrawn LNG to the body of LNG in the reservoir at a rate sufiicient to oiTset heat leakage into the reservoir and to maintain equilibrium conditions in the reservoir so as to prevent boil-off,

(f) and means for adding N -rich gas from said vapor space to said external refrigerant as make-up gas, when the feed stock contains a high percentage of N References Cited by the Examiner UNITED STATES PATENTS 2,263,864 11/1941 Avigdor 62-45 X 2,682,154 6/1954 Wilkinson 6254 2,753,691 7/1956 Wissrniller -62-54 2,784,560 3/1957 Johnson 62--54 2,959,928 11/1960 Maker 62-54 2,960,837 11/1960 Swenson et a1. 62-45 X LLOYD L. KING, Primary Examiner. 

1. SYSTEM FOR MAINTAINING THE SUBSTITUTABILITY OF LNG COMPRISING (A) A LARGE-SCALE STORAGE RESERVOIR FOR LNG HAVING A VAPOR SPACE NEAR THE TOP THEREOF, (B) MEANS FOR SUPPLYING LNG TO SAID RESERVOIR FOR STORAGE AT SUBSTANTIALLY ATMOSPHERIC PRESSURE, (C) A HEAT-EXCHANGE UNIT IN SAID VAPOR SPACE, SAID UNIT HAVING A SUPPLY OF REFRIGERANT EXTERNAL TO THE RESERVOIR AT A LOWER TEMPERATURE THAN THTE STORED LNG, (D) MEANS FOR WITHDRAWING LNG FROM A POINT NEAR THE BOTTOM OF THE RESERVOIR AND PASSIN IT THROUGH SAID HEAT-EXCHANGE UNIT TO SUBCOOL THE WITHDRAWN LNG, (E) MEANS FOR RETURNING THE SUBCOOLED WITHDRAWN LNG TO THE BODY OF LNG IN THE RESERVOIR AT A RATE SUFFICIENT TO OFFSET HEAT LEAKAGE INTO THE RESERVOIR AND TO MAINTAIN EQUILIBRIUM CONDITIONS IN THE RESERVOIR SO AS TO PREVENT BOIL-OFF. 